This invention is a method to determine remaining battery capacity of a rechargeable battery, and in particular it relates to a method to determine remaining battery capacity by correcting the computed battery capacity to determine an accurate remaining battery capacity.
In portable electronic equipment such as notebook personal computers and portable telephones, rechargeable batteries are used as the power source. For these applications, the remaining battery capacity of the rechargeable batteries is determined and displayed to prevent sudden loss of power, possible data loss, and inability to use the equipment.
This remaining battery capacity is normally determined by integrating battery discharge current and subtracting the integrated value from the rechargeable battery charge capacity. However, error develops between the remaining battery capacity determined by this method and the actual remaining battery capacity. The error results from rechargeable battery degradation and changes in charge capacity due to variation in the operating environment.
The technology to compensate for this error and display an accurate remaining battery capacity is disclosed in Japanese Non-examined Patent Publication No. 5-87896 issued Apr. 6, 1993. In this disclosure, techniques are cited to correct the remaining battery capacity determined from integrated discharge current to find an accurate remaining battery capacity based on previously established actual remaining rechargeable battery capacity corresponding to a plurality of specified voltages such as the fully charged battery voltage, the low battery voltage, and the dead battery voltage.
However, even with this type of compensation, inability to obtain an accurate remaining battery capacity is possible. FIG. 2 is a graph showing rechargeable battery (specifically, nickel cadmium rechargeable battery) voltage change during discharge. The solid line in FIG. 2 is the discharge curve for the rechargeable battery in its initial state, and the broken line is the discharge curve of the rechargeable battery after a specified number of discharge cycles.
Clearly from this graph, the rechargeable battery discharge curve drops as the number of discharge cycles progresses. Consequently, when the measured remaining battery capacity is corrected by a previously established remaining battery capacity corresponding to a specified voltage (for example, the first voltage V1 in FIG. 2), a large disparity develops as the number of discharge cycles progresses.
Specifically, the remaining battery capacity at the point where the battery voltage of the rechargeable battery represented by the broken line discharge curve reaches the first voltage V1 is clearly greater than the remaining battery capacity of the rechargeable battery represented by the solid line at voltage V1. Regardless of this, the remaining battery capacity of the battery represented by the broken line discharge curve is corrected to the previously established remaining battery capacity (solid line discharge curve) corresponding to the first voltage V1. Therefore, a remaining battery capacity which is less than the actual remaining battery capacity results. The present invention was developed to overcome this type of shortcoming, and an object of the present invention is to provide a method which can always accurately determine remaining battery capacity of a rechargeable battery.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.